1. Field of the Invention
The present invention relates to a transfer apparatus for transferring a web from an upstream dryer to a downstream dryer of a dryer section. More particularly, the present invention relates to a transfer apparatus for transferring a web without open draw between a first and a second felt of a dryer section.
2. Information Disclosure Statement
Co-pending U.S. patent application Ser. No. 07/014,569 to Beloit Corporation discloses a TOTAL BELRUN dryer section in which a web is guided from a press section to a calender through a dryer section wherein opposite sides of the web are alternately dried as the web moves without open draw between successive tiers of dryers. TOTAL BELRUN is a registered trademark of Beloit Corporation.
In a double felted prior art proposal, a web is dried by passing the web in sinusoidal configuration past a plurality of dryer drums such that a top felt presses the web into close conformity with an upper tier of dryers and a bottom felt presses the web into conformity with the heated external surface of a lower tier of dryers. Of necessity, such a dryer arrangement requires the web to progressively move between the dryers of the respective upper and lower tiers in open draw. Accordingly, movement of the web in such open draws causes edge flutter, curl and cockling of the resultant web.
In order to overcome the problem of edge flutter and the like, the so-called serpentine run dryer section was developed in which the web was supported throughout a dryer section by a single dryer felt moving in sinusoidal configuration around the dryer drums of the upper and lower tiers. Although the serpentine configuration helped to alleviate the problem of edge flutter, a further problem was introduced in that the lower dryer drums became redundant because the dryer felt disposed between the web and the lower dryer drums acted as an insulator and prevented drying of the web during passage around the lower dryer drums.
In the TOTAL BELRUN arrangement of patent application Ser. No. 07/014,569, the web and dryer felt move contiguously around alternately a dryer drum and a vacuum transfer guide roll. The arrangement is such that the web is held in close conformity with the dryer felt during passage of the web around the vacuum transfer roll. Also, the web is sandwiched between the dryer felt and the dryer drum during passage of the web around the dryer for drying thereof. Furthermore, the web is transferred without open draw between successive tiers of dryers so that as the web moves between successive tiers, opposite sides of the web are dried. Accordingly, as the web progresses from the press section to the calender, opposites sides of the web are alternately dried and the web is restrained against cross-machine shrinkage thereof. Therefore, edge flutter, curl and cockling of the web are inhibited.
The present invention is an improvement over the arrangement disclosed in U.S. patent application Ser. No. 07/014,569 and relates particularly to the transfer arrangement between successive tiers. In patent application Ser. No. 07/014,569, an upstream and a downstream vacuum roll guide the respective dryer felts such that the dryer felts define therebetween a web transfer section. However, vacuum transfer rolls are relatively costly, and the present invention seeks to provide a transfer apparatus without open draw of the web using plain transfer rolls in association with adjacent web transfer boxes.
Therefore, it is a primary object of the present invention to provide a transfer apparatus which overcomes the aforementioned disadvantages of the prior arrangement and which provides a significant contribution to the art of drying a web extending through a dryer section.
Another object of the present invention is the provision of a transfer apparatus which includes a first felt roll and a second felt roll disposed between an upstream dryer and the first felt roll such that the web is disposed between the second felt roll and the first felt. A second dryer felt passes around the second felt roll and extends from the second felt roll to and around a downstream dryer such that the first and the second felts defines therebetween a web transfer section, the web being sandwiched between the first and the second felts between the second and the first felt rolls.
Another object of the present invention is the provision of a transfer apparatus which includes means for positively transferring the web from the first to the second felt when the first felt diverges relative to the second felt.
Another object of the present invention is the provision of a transfer apparatus in which a first and second felt roll and an upstream and downstream dryer are disposed relative to each other such that the web between the upstream and downstream dryers follows a common tangent to the dryers and felt rolls.
Another object of the present invention is the provision of a transfer apparatus in which the first and the second felt rolls intersect a common tangent of the upstream and downstream dryers such that the first felt wraps around the second felt roll and the second felt wraps around the first felt roll so that the felts extending between the second and first felts rolls are disposed parallel relative to each other for stabilizing the felts during transit through the transfer section.
Another object of the present invention is the provision of a transfer apparatus in which the felts define therebetween a converging nip such that when the second felt is driven at a higher speed relative to the first felt, tension within the web during transfer between the first and the second felts is maintained.
Another object of the present invention is the provision of a transfer apparatus which includes a blow box disposed between the first felt roll and the downstream dryer for generating an upstream flow of air for drawing the web into close conformity with the second felt when the first felt diverges relative to the second felt. The blow box also prevents air from entering between an upstream orifice of the blow box and the first felt roll in a direction from the second felt roll which would otherwise tend to cause a tail of the web to follow the first felt.
Another object of the present invention is the provision of a transfer apparatus which includes a blow box having a downstream orifice such that during threading of the dryer section, a tail of the web is positively held in close conformity with the second felt until the tail is disposed between the downstream dryer and the second felt.
Another object of the present invention is the provision of a transfer apparatus including a blow box having openings along the edges thereof so that during use of the apparatus, air flows through the openings for drawing the first and the second edges respectively into close conformity with the second felt so that edge flutter of the web relative to the second felt is inhibited.
Another object of the present invention is the provision of a transfer apparatus including a vacuum box for drawing the web into close conformity with the second felt when the firs felt diverges relative to the second felt and for preventing air from entering between an upstream slot and the first felt roll in a direction from the second felt roll.
Another object of the present invention is the provision of a transfer apparatus which includes a plenum for generating a current of air such that the web is drawn into close conformity with the first felt when the first felt diverges relative to the upstream dryer and for diverting a boundary layer of air that follows the upstream dryer.
Another object of the present invention is the provision of a transfer apparatus which includes a plenum for directing a current of air such that during threading of the dryer section, a tail of the web is positively urged into close conformity with the first felt until the tail is sandwiched between the felts during passage through the web transfer section.
Another object of the present invention is the provision of a transfer apparatus in which the transfer means defines a wall which diverges in a direction from the first felt roll towards the downstream dryer so that during movement of the second felt between the first felt roll and the downstream dryer, a vacuum is generated between the wall and the second felt for drawing the web into close conformity with the second felt.
Another object of the present invention is the provision of a transfer apparatus in which the plenum includes a surface which is disposed on the opposite side of the first felt relative to the second felt roll for generating a partial vacuum between the surface and the first felt for urging the web into close conformity with the first felt between the upstream dryer and the second felt roll.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration o the detailed description contained hereinafter taken in conjunction with the annexed drawings.